1. The Field of the Invention
The present invention relates to polyether compounds usable as solid propellant bonding agents and having imine, or imine and hydroxyl functionality. More particularly, the present invention relates to the use of such organic compounds as bonding agents in the preparation of solid propellants.
2. Technical Background
Solid propellants are used extensively in the aerospace industry. Solid propellants have become the preferred method of powering most missiles and rockets for military, commercial, and space applications. Solid rocket motor propellants have become widely accepted because of the fact that they are relatively simple to manufacture and use, and they have excellent performance characteristics. Furthermore, solid propellant rocket motors are generally substantially more simple and reliable than liquid fuel rocket motors. For all of these reasons, it is found that solid rocket propellants are very reliable and economical.
Typical solid rocket motor propellants are formulated using an oxidizing agent, a fuel, and a binder. At times, the binder and the fuel may be the same. In addition to the basic components, it is conventional to add various bonding agents, plasticizers, curing agents, cure catalysts, and other similar materials which aid in the processing and curing of the propellant. A significant body of technology has developed related solely to the processing and curing of solid propellants.
Many types of propellants used in the industry incorporate ammonium perchlorate (AP) as the oxidizer. The ammonium perchlorate is generally incorporated into the propellant in particulate form. In order to hold the propellant in a coherent form, the components of the propellant are bound together by binder, such as, but not limited to, a hydroxy-terminated polybutadiene (HTPB) binder. Such binders are widely used and commercially available. It has been found that such propellant compositions provide ease of manufacture, relative ease of handling, good performance characteristics, and are at the same time economical and reliable. As a result, this type of propellant has become a standard in the industry.
Propellants are generally required to meet various mechanical and chemical performance criteria in order to be considered acceptable for routine use. For example, it is important that the propellant have desired mechanical characteristics which allow it to be used in a corresponding rocket or missile. It is important, for example, that the propellant elastically deform during use in order to avoid cracking within the propellant grain.
If the propellant cracks, burning within the crack may be experienced during operation of the rocket or missile. Such burning in a confined area may result in an increased surface area of burning propellant or increased burn rate at a particular location. This increase in the burn rate and surface area can directly result in failure of the rocket motor due to over pressurization or burn through of the casing.
Accordingly, propellants are typically subjected to standardized stress and strain tests. The typical configuration of the propellant sample tested is often referred to as a JANNAF Class C specimen. The shape and size of such specimens are standard in the industry. Such specimens are typically placed in an INSTRON® testing apparatus and then pulled until the specimen fails. Data is recorded during such tests and objective measures of stress and strain performance are provided.
In order to make certain that propellant formulations meet the applicable specifications, it is often necessary to employ a bonding agent within the propellant composition. Bonding agents are used in order to help incorporate solid particles into the polymeric binder system. Use of a bonding agent typically improves the stress and strain characteristics of the propellant.
A number of bonding agents are known and conventional. One such bonding agent is Tepanol (tetraethylenepentamine acrylonitrile glycidol adduct). Tepanol has been found to be useful as a bonding agent, and improves the processing characteristics of the propellant formulation. Tepanol is believed to become chemically linked to the polymeric propellant binder. Tepanol also electrostatically coordinates with the remaining ammonium perchlorate after forming a Tepanol perchlorate salt from an acid/base reaction with ammonium perchlorate. Thus Tepanol aids in binding the ammonium perchlorate particles within the propellant matrix. Tepanol is also inexpensive and readily available.
Tepanol, however, also causes difficulty in the formulation of propellant. Tepanol is relatively basic, and in the presence of ammonium perchlorate produces a significant amount of ammonia. This makes it necessary to conduct propellant mixing steps under vacuum, and to mix for long periods of time in order to substantially remove the produced ammonia. These characteristics of Tepanol result in significant disadvantages, such as long mix time, high labor costs, ammonium perchlorate attrition, and may shorten the service life of the propellant.
An alternative bonding agent is known commercially as HX-752 and is available from 3M. HX-752 is an aziridine having the following general chemical structure:

HX-752 is believed to be incorporated into the propellant matrix by ring opening polymerization. HX-752 avoids the production of large amounts of ammonia which plague processes using Tepanol. As a result, some advantages are derived from the use of HX-752.
Even in view of the foregoing, HX-752 is far from ideal as a bonding agent. One significant problem is that of economics. HX-752 presently costs from four to five times as much as Tepanol. Also, the propellant produced when using this material has a relatively high viscosity, which inhibits processing. It is also believed that HX-752 may be a carcinogen. Thus, it can be seen that the cost and chemical characteristics of HX-752 make it a less than ideal bonding agent.
In summary, conventional bonding agents have significant drawbacks. Tepanol is problematic because of its tendency to produce large quantities of ammonia during propellant mixing and the other limitations mentioned above. Alternative materials, such as HX-752, also present problems including cost and the processing characteristics of the propellant.
Accordingly, it would be an advancement in that art to provide bonding agents which overcame some of the significant limitations encountered using conventional bonding agents. It would be an advancement in the art to provide bonding agents which did not produce significant quantities of ammonia during propellant formulation. It would also be an advancement in the art to provide acceptable alternative bonding agents which were relatively inexpensive. It would also be an advancement in the art to provide such bonding agents which also resulted in propellants having acceptable stress and strain characteristics.
Such compositions and methods are disclosed and claimed herein.